FIG. 4 illustrates a configuration of a typical multiband compressor. The multiband compressor provides a sound process of splitting an inputted audio signal with a band splitting filter (also referred to as isolator) by frequency bands, applying a compressor process for each of the split frequency bands, synthesizing the split audio signals, and outputting the synthesized audio signal.
The compressor process provides a function of measuring levels of an inputted signal, subjecting the inputted signal to a weighting process in accordance with a sound level curve as illustrated in FIG. 5, and outputting the subjected signals. A boundary level of an input signal called a threshold is given in advance, and the sound level is reduced in accordance with the designated amount of compression when the sound level more than or equal to the threshold is inputted.
By applying the output gains by the amount corresponding to the reduction in the sound level from the maximum input, it is possible to obtain an effect of enhancing the average sound level throughout the entire compressor. The multiband compressor performs the compressor process separately for each frequency band. Thus, in the case where the sound level of one frequency band suddenly changes, this sudden change does not have any effect on the other frequency bands.
In the case where a sound contains plural sound sources (general music), it is possible to avoid unnatural sound fluttering, and to sufficiently obtain the effect of enhancing the sound level. As the multiband compressor as described above, there have been various propositions (see, for example, Patent Documents 1 to 3).